1. Field of the Invention
The present invention relates to a method for accurately mixing dry cement with water to form a cement slurry having a desired density.
2. Description of the Prior Art
In tile past, cement slurries have been continuously produced by mixing accurately metered water and dry powdered cement. The water is easily metered by, for example, a valve and a flowmeter. The dry cement is very difficult to meter and is metered by, for example, allowing fluidized cement to flow from a pressurized surge can through a knife gate or butterfly valve. The water and dry cement flow into a mixing means to yield a cement/water slurry.
Many cement job failures are caused by a lack of adequate, steady delivery of bulk cement to the cementing unit during the job. The job failures caused by flow interruptions, plugging of tools by chunks of set cement, and erratic flow result in poor primary cement jobs, many of which require remedial cementing jobs. A better-controlled flow of dry cement would help prevent this type of failure, thereby reducing the number of remedial cement operations.
The use of recirculating cement units equipped with automatic density control systems has drawn attention to pneumatic bulk supply systems. The automatic density control system puts out a post-job record showing cement density throughout the job and shows when flow has been interrupted or becomes erratic.
On rigs having a history of erratic cement delivery, pressurized separator tanks are often included in bulk delivery systems to provide a steady flow of dry cement. A nonpressurized version of the separator can be used to feed (by gravity) a cement hopper for jet mixing systems.
Many methods have been developed to accurately meter the dry powdered cement. However, at best, the flow of dry cement into the mixer is variable and consequently the cement/water slurry produced has a variable density. Additional steps, e.g., averaging tanks must be used to obtain a cement/water slurry having a constant density.
A cement mixing method which can accurately and continuously produce a cement/water slurry would produce better quality cement, eliminate the need for averaging tanks and greatly reduce the amount of waste cement/water slurry generated at job sites.
The continuously produced cement/water slurry must have a desired density and must have a minimum degree of variation in density, preferably .+-.0.1 pounds of cement per gallon or less.